Any device plugged into the electric AC grid and requiring DC power needs a power supply comprising an AC-DC converter with power factor correction (PFC). Examples include, but are not limited to, a battery charger, a telecommunication device, a computing device and an Uninterruptible Power Supply (UPS).
As a specific example, consider an electric vehicle propelled by an electric motor that is supplied with power from a rechargeable battery. The rechargeable battery can be recharged using an AC-DC charger. A proper topology selection for the AC-DC charger with PFC is essential to meet the regulatory requirements of input current harmonics, output voltage regulation and implementation of PFC. Hence, the power supply industry has developed many AC-DC converters with PFC.
FIG. 1 is a circuit diagram of a single-stage boost converter 100 of conventional design, comprising a diode bridge rectifier 110. The single-stage boost converter is the most widely used AC-DC converter for PFC applications. The single-stage boost converter is very simple and achieves near unity power factor if proper control techniques are used. However, the single-stage boost converter has some drawbacks, including high conduction losses and therefore heat management issues in diode bridge rectifier 110.
Unlike the single-stage boost converter of FIG. 1, there is no diode bridge rectifier in a dual boost converter. The dual boost converter reduces the total semiconductor count from six to four, and reduces the losses and associated heat management issues in the diode bridge rectifier. The input voltage in the dual boost converter is floating with respect to ground, and exhibits high common-mode (CM) noise.
FIG. 2A is a circuit diagram of a semi-bridgeless converter 200a of conventional design. It reduces CM noise by adding two slow diodes, D3 and D4, to the dual-boost converter. Semi-bridgeless converter 200a comprises two PFC inductors Lin1 and Lin2. Since the return path inductance conducts only a small portion of the total return current, the total converter inductance of the semi-bridgeless converter of FIG. 2A is twice that of the single-stage boost converter of FIG. 1.
FIG. 2B is a circuit diagram of an embodiment of a totem-pole converter 200b. The totem-pole converter 200b overcomes the CM noise issue without adding extra diodes and it does not need the two PFC inductors as the semi-bridgeless converter 200a does. However, the totem-pole converter 200b uses a MOSFET intrinsic body diode to carry the load current, creating a reverse-recovery problem, which makes it unfavorable for use in continuous conduction mode (CCM), high power applications. To reduce the reverse-recovery losses of the body diode, other topologies have been proposed, but their practical implementation is complex.
FIG. 2C is a circuit diagram of an embodiment of a regulated DC to DC converter 200c (see, for example, U.S. Pat. No. 4,559,590). DC to DC converter 200c is an example of an isolated DC to DC converter with zero voltage switching capability. DC to DC converter 200c can be suitable for high voltage, multiple outputs.
In addition to the drawbacks noted for the aforementioned AC-DC topologies (FIGS. 1, 2A and 2B), the circuits inherently have high in-rush current and a lack of lightning and surge protection owing to the direct connection between the AC input voltage and the bus capacitors, through the PFC diode and PFC inductor. Hence, for practical applications the converters described above require in-rush current and surge limiting to prevent damage when a connection is made to AC power.
For high efficiency PFC converters of several hundred Watts and greater (>400 W), in-rush current and surge limiting is typically achieved by placing a current limiting device (for example a resistor or positive-temperature-coefficient device) in series with the PFC circuit and shorting the current limiting device out with a relay after the difference between the bus voltage and peak rectified AC input becomes sufficiently small.
Surge limiting circuits, such as the one described above, add cost, complexity, and often regulatory difficulties when requiring voltage sensing and control of the relay crossing isolation boundaries (for example if there is an isolated DC-DC converter followed by the PFC converter). Surge limiting circuits also need to be tolerant of AC power brownouts and blackouts which again add cost and complexity.
Consider a scenario where an AC surge limiting resistor has been shorted out by the relay and the AC power drops out for several cycles, preventing the PFC converter from maintaining the PFC bus voltage. During this short time, the DC bus-voltage (output voltage of the PFC converter) will drop while it supplies downstream loads. When AC power is restored, the AC input relay will still be shorting out the surge limiting resistor and can allow a potentially damaging surge current to flow through the PFC components and eventually damage the power devices. For this reason, and other related AC power quality issues, a robust surge limiting implementation can be complex and expensive.
While there are examples of converters that limit the in-rush current, those converters are restricted to low power applications (<400 W). FIG. 3 shows an example of a bridgeless converter 300 of conventional design. Bridgeless converter 300 has been configured so that it does not suffer from high in-rush start-up current. Moreover, the implementation of lightning and surge protection is easier compared to the single-stage boost converter of FIG. 1.
Bridgeless converter 300 comprises two pulse width modulation (PWM) switches S1 and S2 (i.e., switches driven via PWM signals). The major drawback of bridgeless converter 300, however, is the voltage spike across the PWM switches S1 and S2 for high frequency and high power applications. As a result, bridgeless converter 300 is restricted to low power applications. Furthermore, bridgeless converter 300 may have other limitations such as only working with complex digital control implementation and variable frequency operation.
Another drawback of bridgeless converter 300 is that the positive AC line cycle and the negative AC line cycle operations are not symmetric, which can increase the complexity of the converter control system.
There is a need for an AC-DC PFC converter that is suitable for use in power supplies for devices connected to the AC grid and that can be configured to overcome the previously stated disadvantages.